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My guess is a light tub with rough finish could go the longest between scrubs.
I agree with the other comments. Its all what you like. On my old tub I had a textured finish which was not as slick as my new smooth finish tub. Both are just as easy to clean, however with my smooth finish tub now, I notice a water line around the tub, where I didn't on my old tub (probably because the textured one was colored and my smooth one is pearl white). If you go with a smooth finish, be careful when draining it. I used a sumpump to drain my tub a couple weeks ago, and noticed a couple scratches on the floor surface.
There SHOULD NOT be a scum line on ANY tub that is properly maintained so you can immendiately discount that concept as any type of deterant.
"SHOULD NOT" and "REALITY" sometimes don't play well together, so don't poo-poo the idea just yet....I had a water line, but it wasn't scum, it was just from the hard water. It comes off, at any rate....but with a pearly finish, it is visible.
I use those Mr Clean Magic Erasers.